THE CHEMISTRY Reactions that life depends on need a place to happen. That place is the cell. All that biology recognizes as undeniably alive are either cells or cell conglomerates (viruses fall into disputed territory). The cell has been the basic unit of life since the middle of the 19th century.
Enjoy more audio and podcasts on iOS or Android.
A cell needs something to keep the inside inside and the outside outside. This is the job of the cell membrane, a flexible film made largely of lipids. These are small, tadpole-shaped molecules with heads that are comfortable in water and twin tails that avoid it. When placed in an aqueous solution, they naturally form double layers with the water-tolerant heads on the outside and the water-repellent parts on the inside. Some plant, fungal, and bacterial cells use more rigid structures called cell walls as additional attachments beyond their membranes. But it is the membrane that defines the cell.
In addition, the arrangement of the membranes determines what type of cell it is. Some creatures use membranes primarily to define their girth. These are called prokaryotes and come in two types, bacteria and archaea. In others, they are also used to create structures within cells, specifically a nucleus that contains the DNA on which genes are written. Such cells can have ten or twenty times more membranes in them than they define their surfaces. They are called eukaryotic, Greek for really nucleated. Creatures from them are eukaryotes.
The prokaryotic cells of the world outnumber their eukaryotic cousins ââby far. There are roughly as many unicellular prokaryotes (mostly in the intestines) in your own body as there are eukaryotic cells that make up muscles, nerves, bones, blood, etc. Some parts of the Earth’s biosphere, such as the ocean floor, contain more or less nothing but prokaryotic life.
But almost everything you’ve ever seen and recognized as alive – all animals, plants, fungi, and algae – is made up of eukaryotic cells. Such cells are typically much larger than almost all prokaryotic cells and are able to achieve a much greater variety in both form and function. Their versatility is evident in the wide range of shapes they take, from the connected star explosions of nerve cells to the creeping, mutable blobs of amoeba.
But prokaryotic cells are also large compared to the molecules they contain. A bacterium two millionths of a meter long contains around 3 million protein molecules as well as the DNA what she describes that RNA necessary to take advantage of these descriptions and the various smaller molecules that proteins stick together and break apart in the course of their tasks (see previous biology briefing). The membrane of such a bacterium also contains around 20 million lipid molecules.
But if you were to synthesize all of the molecules found in this bacterium in a laboratory (quite possible in theory) and put them in a bacteria-sized bag, you wouldn’t get a bacterium. You’d get a little mess. A cell is not just a set of content. It’s also a series of processes that run side by side. The only way to create a cell that has all of the necessary processes running is to start with another cell where they are already doing this.
Feed a bacterium the nutrients it needs and as it grows it synthesizes a copy of the DNA Molecule on which its genome is stored. If it’s big enough to have made a full copy of it DNA it will split in two, with one DNA ends up in one cell and the other in the other.
As it is with bacteria, so it is mutatis mutandis, for all other life, forever and ever, amen. Life is made up of cells and cells are made up of cells that already exist. The 30 trillion cells that make up a human body can in almost all cases be traced back to the only fertilized egg that started it all (with the exception of so-called chimerism, in which two embryos fuse in the womb in the early stages of development). .
Of all the processes that continue from cell to cell in the course of life, none is more fundamental than the one that supplies life energy. These are completely dependent on the membranes in cells. The conditions on the two sides of a membrane are almost always different; different molecules will be present in different concentrations. The laws of thermodynamics, however, see different concentrations of something side by side. Small molecules and ions, which are more abundant on one side of this membrane than the other, diffuse through it to balance things out. Proteins embedded in such membranes pump molecules in the opposite direction to maintain the distinction between inside and outside.
By building a gradient of hydrogen ions – hydrogen atoms with withdrawn electrons – across a membrane, living beings bring energy into a chemical form that they can use. This process depends on a number of proteins called electron transport chains. These proteins are embedded in the membrane.
Electron transport chain proteins pass electrons on to one another in such a way that hydrogen ions on the inside of the membrane are moved outwards. So the ions build up outside, which means that nature’s tendency to equalize concentrations requires some of them back inside. They do this with the help of a great protein called ATP Synthase, or just ATPass. Molecules of ATPThese provide channels through the membrane through which the hydrogen ions can easily flow. This flow provides usable energy, like the flow of water through a watermill.
This is not an empty metaphor. ATPase consists of several parts, one of which can rotate against the other. As the ions flow through the protein, they turn this rotor at a speed of 6,000 rpm. If you could hear them at work, they’d hum something like the G two octaves below middle C. Another part of the molecule uses the kinetic energy of this spinning rotor to add phosphate ions to a molecule called adenosine diphosphate (ADP), which creates adenosine triphosphate, or ATP– the almost universal energy source in cell biology.
In almost all cases where a cellular process requires energy, this energy is provided by breaking ATP back down in ADP. Adding an amino acid to a growing protein uses roughly five ATPS. The synthesis of membrane lipids costs about one ATP for every two carbon atoms used. A bacterium that doubles in size uses around 10 billion ATPs build all the molecules needed, ie each of the 10m or so ADP Molecules that the bacterium contains will be in ATP and dismantled 1,000 times during the process.
To keep them ATPWhen buzzing, the cell needs a constant flow of electrons along its membrane-bound electron transfer chains. There are two ways to create such flows: respiration and photosynthesis.
Breathing breaks down glucose molecules into carbon dioxide and water through a series of reactions called the citric acid cycle. The electron value of a glucose molecule typically pushes ten hydrogen ions through the membrane in which the respiratory electron transfer chain is embedded. When they flow back through that ATPsince she was 20. can generate ATPS.
Photosynthesis uses the energy of sunlight to release electrons from water molecules, creating oxygen and also hydrogen ions ready to push through the membrane. Some of the ATP This drives a process that combines these ions with carbon dioxide. A few more chemical reactions produce a sugar, like glucose, which is then built into all of the other molecules that make up life. Photosynthesis builds up the world’s biomass; breathing decomposes it.
There was once
In a prokaryotic cell, the membrane that holds electron transfer proteins is the one that surrounds the cell. In eukaryotic cells, breathing takes place in intracellular structures – organelles – called mitochondria. These consist of folded membranes that are rich in electron transport chains. Containing many mitochondria (hundreds or thousands per cell is not uncommon in humans) means that such cells can produce a large amount of mitochondria ATP. If all of the membranes in your body’s mitochondria were connected and spread flat, they would cover several soccer fields.
Some mitochondria look like bacteria under the microscope. This is not a coincidence, but a family resemblance. When the earth was a little more than half its current age, i.e. about 2 billion years ago, two prokaryotes, one from the archaea and one from the bacteria, fused together. How exactly they did this is not yet entirely clear. But this fusion created something really new: the first eukaryotic cell. Mitochondria are descendants of the bacteria involved, an ancestry that is irrefutably proven by the fact that they still have their own residual genomes that are clearly bacterial. In humans, these small mitochondrial genomes are the only ones DNA not secreted on chromosomes in the nucleus.
All mitochondria in all eukaryotes in the world go back to this amalgamation. Similarly, chloroplasts – the organelles of photosynthesis found in plants and algae – go back to a later event in which a eukaryote devoured a photosynthetic bacterium. Many eukaryotes remained unicellular and continue to do so to this day. But others began to form colonies that enabled the division of labor between cells and encouraged the development of specialized parts of the body called organs. What are the topics of next week’s biology briefing? â
In this series about the levels of life
1 The large molecules in biology
2 cells and how to power them
Create 3 organs
4 The story of a life
5 What is a species anyway?
Find 6 living planets
This article appeared in the short section Schools of the print edition under the heading “Strata of Power”